1. Field of the invention
This invention relates to a three-phase thermal protector suitable for opening and closing a neutral of wye-connected phase windings of three-phase induction motors for protecting the motors against overcurrent and overload conditions or suitable for protecting like electric machines.
2. Description of the prior art
Conventional three-phase thermal protectors of the above-described type are disclosed by Japanese Published Utility Model Registration Application No. 31-5747 entitled "DISH-SHAPED BIMETAL RELAY WITH THREE PAIRS OF CONTACTS" or by Japanese Published Patent Application No. 46-34532 entitled "THERMOSTAT SWITCH." Six contacts, that is, three pairs of movable and fixed contacts are employed in each of the above mentioned protectors. Such a number of contacts entails an economic problem that the manufacturing cost of the protector is increased.
The applicants of the present application have disclosed, in Japanese Laid-open Patent Application No. 1-105435, a three-phase thermal protector in which the number of the contacts is reduced to four, that is, two pairs of movable and fixed contacts. However, a screw is employed in an operating temperature calibrating mechanism in this three-phase thermal protector. Provision of the screw requires a high level of accuracy in manufacture of the protectors and increases the number of parts. Furthermore, it is disadvantageous that the deviation in the calibrated operating temperature cannot be corrected after a header plate has been welded to a housing for its hermetic sealing after completion of the calibration of the operating temperature.